Comparison of metabolic rates among macrophyte and nonmacrophyte habitats in streams

Abstract

Little is known about the relative contribution of different stream habitats to reach-scale metabolism. We measured in situ metabolism in sand, gravel, stone, and macrophyte habitats to compare metabolic rates among these habitat types and to compare habitat-weighted measurements with reach-scale measurements. We used open-bottom chambers in sand, gravel, and macrophyte habitats and closed-bottom chambers in stones, and we estimated reach-scale metabolism from 2-station O 2 budgets. Macrophyte habitats had a significantly higher gross primary production (GPP) and community respiration (CR) than stone, gravel, and sand habitats. A large part of this difference was associated with epiphytic biofilm: 28% of net ecosystem production (NEP), 20% of CR, and 24% of GPP. Macrophyte habitats contributed proportionally more to reach-scale metabolism than did the other habitat types. Forty-one percent of reach-scale NEP, 60% of ecosystem respiration (ER), and 50% of GPP were associated with this habitat type even though only 14% of the reach was covered by macrophytes. We found significant linear relationships between GPP and CR and the amount of autotrophic biomass in the streams. The rates reported in 11 literature studies fit into our observed relationships, showing the generality of our findings. The rates we obtained expand the range of reported metabolic values in relation to autotrophic biomass for both low and high biomass. The importance of macrophyte habitat can be ascribed to the macrophytes themselves, the associated epiphytic biofilm, and the fine organic material trapped in the dense stands. We conclude that besides having an effect on the structural elements in streams, macrophytes contribute significantly to stream ecosystem functions.

Riverine macrophytes control seasonal nutrient uptake via both physical and biological pathways

“…We saw that GPP was highest in spring and summer and lower in autumn, whereas ER was low in spring, but high in summer and autumn. Reach-scale N uptake was below detection (BD) in autumn, and heterotrophic assimilation was low in spring (and therefore difficult to visualise on the plot, although the relative contributions are included in the figure) in epiphytic biofilms colonising macrophytes (Alnoe, Riis, & Baattrup-Pedersen, 2016;Levi et al, 2015), while benthic biofilms may be less prevalent as a result of macrophyte shading. This counter-intuitive outcome may be explained by several mechanisms.…”

“…For example, 24% of primary production in a macrophyte habitat was due to epiphytic microalgae (Alnoe et al, 2016); this contribution can be even higher when macrophytes themselves are light-limited, and their role as substrate for epiphytic biofilm increases. Therefore, in this study, the difference in spring and summer GPP might have been due to the increased role of benthic biofilm plus macrophytes in spring, whereas the fully developed epiphytic and benthic biofilm, without significant macrophyte contribution, in summer may have resulted in lower reach-scale GPP.…”

Using the raz‐rru method to examine linkages between substrate, biofilm colonisation and stream metabolism in open‐canopy streams

“…Raz transformation was also positively correlated with chlorophyll a and GPP, indicating that the contribution of autotrophic respiration to ER increased predictably with algal biomass. A recent meta‐analysis similarly reported that respiration increased with autotrophic biomass and GPP (Alnoee, Riis, & Baattrup‐Pedersen, ).…”

Impacts of water level on metabolism and transient storage in vegetated lowland rivers: Insights from a mesocosm study

“…Likewise, aquatic vegetation abundance may stimulate heterotrophic metabolism by providing substrate for epiphytic biofilms in the water column, and by supplying and trapping organic matter. A literature review by Alnoee et al [] showed positive correlations between autotrophic biomass and primary production and respiration, while O'Brien et al [] found that primary production, but not ecosystem respiration, was closely correlated with macrophyte cover. Our results of daily rates of GPP agree with these findings, with higher rates in mesocosms with higher water depths and coverage of aquatic vegetation (Figure ).…”

“…Unfortunately, our study is not able to directly distinguish the relative importance to observed metabolic activity of the contribution of the aquatic vegetation itself from that of associated epiphytic biofilms, nor the relative importance of the vegetated surface water from that of hyporheic sediments. Nevertheless, stream sections dominated by aquatic vegetation have been shown to have higher metabolic activity than those dominated by sediment, in large part due to epiphytic biofilms associated with the vegetation [ Alnoee et al ., ]. Given the high surface area of Ranunculus , the dominant vegetation type in our mesocosms, the contribution of epiphytes to observed metabolic activity could likewise be high.…”